DESCRIPTION (Applicant's abstract reproduced verbatim): Studies in mice with targeted deletions of neurotrophins or their corresponding receptor tyrosine kinases (trks) have demonstrated that neurotrophin/trk signaling is essential for the survival of most classes of neurons in the PNS during embryonic development. Because of this absolute survival dependence, however, it has been impossible to evaluate the requirement for neurotrophins in regulating other features of neuronal development such as axon growth, target innervation and neuronal differentiation. Increasingly, it has become apparent that neurotrophin survival dependence is regulated by the proapoptotic BCL-2 homologue BAX. Naturally occurring cell death is virtually eliminated in the dorsal root ganglia (DRG) of BAX-deficient mice, and DRG neurons from BAX-deficient mice survive indefinitely in vitro in the absence of any neurotrophin. Bax-deficient mice, therefore, provide a unique system in which to separate the survival requirement for neurotrophins from their effects on axon growth and differentiation in vivo. To this end, we propose to cross BAX-deficient mice with mice carrying a targeted deletion of NGF or its receptor tyrosine kinase, trka. Preliminary studies in Bax-l-,trkA-1- mice have demonstrated that the elimination of BAX allows all DRG neurons to survive in the absence of NGF/trkA signaling in vivo. These findings will be verified in Bax-l-,NGF-1- mice maintained on a pure C57Bl/6 genetic background. We will then study the requirement for NGF/trkA signaling in regulating sensory axon growth, targeting and arborization in peripheral and central targets, using mice in which the trka locus has been replaced by the coding sequence for the axonal marker tau-LacZ. Furthermore, to determine if NGF/trkA signaling is required for the development of specific sensory neurons phenotypes, we will assess whether Bax-deficient DRG neurons express neuropeptides in the absence of NGF in vivo and in vitro. Finally, we propose to test the feasibility of using total RNA from Bax-deficient mice to profile gene expression regulated by NGF/trkA signaling with global expression monitoring.